The aims of our program are to cycle hybrid cells between mouse tetratocarcinoma cells and various kinds of differentiated rodent and human cell lines through mice via blastocyst injection in order to trace histochemically their cell lineage and to study the mechanisms controllling gene activity during mammalian development. The in vitro system of integrating genes, chromosome fragments and entire chromosomes into teratocarcinoma cells in combination with the bioassay of introducing these hybrid cells into the living organism allow us to assay for the appearance, coexistence, and regulation of the foreign gene products in the different organs of chimeric mice. We aim to establish (1) whether it is possible to channel the differentiation of the hybrid cells into specific tissues, (2) if nuclear or cytoplasmic factors derived from the parental differentiated cells are responsible for the tissue preference of the hybrid cells and (3) what specific chromosomes are responsible for this phenomenon. We have introduced the gene for mouse lambda immunoglobulin chain into the pBR 322 plasmid EcoRl site of our pHSV106 vector containing the 3.4 Kb Bam Hl DNA fragment containing the Herpes Simplex virus type 1 thymidine kinase gene. We intend to transform thymidine kinase deficient mouse teratocarcinoma cells with such recombinant DNA molecules and inject the transformants into mouse blastocysts in order to examine the chimeric tissues for the expression of the already rearranged lambda chain DNA sequences. If the exogenously introduced lambda chain gene is expressed in B cells we will introduce deletions and insertions in flanking, coding and intervening DNA sequences of the lambda chain DNA and repeat the transformation experiments. Injection of cloned mouse lambda immunoglobulin chain DNA with and without vector into fertilized mouse eggs should facilitate its chromosoamal integration and transmission through the germ line of the host organism. This novel approach will allow us to correlate the molecualr structure of defined hereditary elements with its developmental phenotype.